Synchronous digital communications system and control installation

ABSTRACT

The network elements NE 1 , . . . , NE 6  of a synchronous digital communications system must be synchronized with each other without the possibility of clock loops occurring. A network node NODE contains network elements NE 1 , . . . , NE 6  and a central clock generator SASE. The network elements transmit clock signals  2 M to the clock generator, which contain a clock of a message signal STM-N and a quality indicator (SSM) contained in the message signal which reflects the accuracy of the clock. The clock generator selects one of the clock signals  2 M as the reference clock REF and informs the control installation STE which of the clock signals it has selected and the degree of accuracy of this clock signal. On the basis of this message STAT the control installation gives instructions ANW to the network elements with respect to the quality indicator which the network elements are to transmit to their outputs.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. Ser. No. 08/994,529filed Dec. 19, 1997 now U.S. Pat. No. 6,163,551 from which priority isclaimed under 35 U.S.C. §120 and under 35 U.S.C. §119 to applicationSer. No. 196 53 261 filed Dec. 20, 1996 in the Federal Republic ofGermany.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention concerns a synchronous digital communications system, acontrol installation for a network node of a synchronous digitalcommunications system, a network element and a central clock generatorfor a network node of a synchronous digital communications system.

2. Discussion of Related Art

A synchronous digital communications system is for example an SDH(Synchronous Digital Hierarchy) system or an SONET (Synchronous OpticalNetwork) system. When messages are transmitted via such a communicationssystem, it is essentially necessary that all the network elements (e.g.Add/Drop-Multiplexer, Crossconnects or Line Multiplexer) operatesynchronously with each other. ITU recommendation G.803 also states thata network node containing several network elements must be supplied witha reference clock by a single central clock generator of the networknode.

A conference article by Dr. M. Wolf, 8th. European Frequency and TimeForum 9., Nov. 3, 1994, TU Munich, pages 166 to 174, describes how thesynchronization of the network elements can take place in such asynchronous digital communications system: One network element receivesmessage signals at two inputs, from which a clock is derived and can beused for the synchronization. The accuracy of the clocks in thesemessage signals is provided by a contained quality indicator, hereaftercalled SSM (Synchronization Status Message, see e.g. ITU-Trecommendations G.707, G.708 or G.709). By means of the SSM a selectiondevice in the network element selects one of the clocks as the referenceclock and routes it to a central clock generator of the network node.This central clock generator distributes the reference clock to all thenetwork elements of the network node. It also describes that the centralclock generator can be an independent unit, or that the clock generatorof a network element, preferably that of a crossconnect, can be used asa central clock generator. To prevent the formation of synchronizationloops, i.e. where two network elements use each other as mutualreference clock sources, an additional SSM is defined: “Do Not Use forsynchronization”, hereafter called DNU. It is transmitted from a networkelement to all the outputs that are connected to the network elementwhich was selected as the reference clock source.

One problem which is linked to the represented solution is that thenetwork elements of the network node do not know the origin of theselected reference clock, or its accuracy. Nor can the central clockgenerator provide this information because the clock is selected by theselection device of a network element. As the accuracy of theirreference clock, the network elements can only indicate in the SSM theaccuracy of the central clock generator during open unsynchronizedoperation, which as a rule corresponds to the accuracy established inthe ITU-T G.812. Also, the SSM DNU cannot be transmitted to the outputswhich are connected to the reference clock source used for thesynchronization, since this reference clock source is not known to thenetwork elements. This can lead to the formation of synchronizationloops.

SUMMARY OF INVENTION

The object of the invention is to provide a synchronous digitalcommunications system whereby the transmission of a quality indicator(SSM) is made possible as a function of the accuracy of the referenceclock source used in the network node. Another object of the inventionis to indicate a control installation for a network node of such acommunications system. Further objects of the invention are to indicatea network element and a central clock generator for such acommunications system.

The object regarding the communications system is fulfilled, accordingto a first aspect of the invention, by at least one network node, whichcontains the network elements, and a central clock generator, and withat least one control installation wherein the network elements areconnected to the central clock generator for receiving a reference clockfrom the central clock generator, the central clock generator isconnected to at least a part of the network elements for receiving atotal of at least two clock signals, each containing a clock and aquality indicator that is indicative of accuracy of the clock, thecentral clock generator is connected to the control installation forsending messages about its synchronization status to the controlinstallation, and where the control installation is connected to thenetwork elements for sending instructions to the network elements, wherethe instructions include quality indicators for transmission by thenetwork elements.

Regarding the control installation for a network node of a synchronousdigital communications system containing several network elements and acentral clock, according to a second aspect of the invention, comprisesmeans for receiving a message about a synchronization status of thecentral clock generator and means for transmitting instructions to thenetwork elements, where the instructions include quality indicators tobe transmitted by the network elements, and which are used to select anexternal reference clock for network elements that do not belong to thenetwork node.

Regarding the network element with a clock provider which can be tunedto a reference clock, a reference clock input that is connected to theclock provider for receiving the reference clock, and a number ofconnector units for receiving and sending message signals, where a clockderivation circuit is assigned to at least a part of the connectorunits, for deriving a clock from the message signal received by thepertinent connector unit, according to a third aspect of the presentinvention, is characterized by clock signal outputs respectivelyassigned to one of the connector units with a clock derivation circuit,for sending clock signals containing the clock of the message signalreceived by the pertinent connector unit, and a quality indicator whichis indicative of the accuracy of this clock, and an input for receivinginstructions, where the instructions include quality indicators to betransmitted by the network element to the individual connector units.

Regarding the central clock generator for a network node of asynchronous digital communications system, with a clock provider whichcan be tuned to an external clock, and a reference clock output fromwhich a reference clock can be obtained for network elements of thenetwork node, such is characterized, according to the invention, by anumber of clock signal inputs for receiving clock signals, each of whichcontains a clock and a quality indicator indicative of the accuracy ofthe clock, a selector unit for selecting the external clock for tuningthe clock provider by means of the quality indicators, and an output forsending a message about the synchronization status of the central clockgenerator to a control installation.

An advantage of the invention is that the formation of synchronizationloops is prevented by a synchronous digital communications systemaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the invention are explained in the following by means ofFIGS. 1 to 4, where:

FIG. 1 is a control installation and a network node comprising severalnetwork elements and a central clock generator,;

FIG. 2 is the network node in FIG. 1 and a two-part controlinstallation;

FIG. 3 is a network element which is connected to a central clockgenerator, and

FIG. 4 is three network nodes and a central control installation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Network elements in a synchronous digital communications system operatesynchronously with each other by tuning their own clock generators to aclock derived from a received message signal. Since network elementsreceive message signals at several inputs, a selection is made from theseveral received message signals as to which clock is to be used to tunethe internal clock provider i.e. which clock is used as the referenceclock. The selection makes use of a quality indicator sent with eachmessage signal, for example as established in the ITU-T recommendationG.707. This quality indicator reflects the accuracy of the clock of therespective message signal and is called SSM (Synchronization StatusMessage) in the following.

In accordance with the invention the selection of a reference clock ismade in a central clock generator, whose object is to provide a centralreference clock to all the network elements of a network node. Anotherimportant point of the invention is that the central clock generatorsends a message about its synchronization status to a controlinstallation. The message contains information regarding the accuracy ofthe clock selected as the reference clock and the origin of this clock.The control installation in turn sends instructions to all the networkelements of the network node regarding which quality indicator they areto transmit from which output.

In a first embodiment illustrated in FIG. 1, a synchronous digitalcommunications system contains a network node NODE and a controlinstallation STE. The network node NODE contains six network elementsNE1, . . . NE6, which are connected via a transmission medium to othernot illustrated network elements of the synchronous digitalcommunications system, and receive and send message signals STM-N. Thisis indicated in FIG. 1 by two double arrows for each network element.However this does not mean that the six network elements NE1, . . . ,NE6 are each connected to two other network elements, but indicates thateach of the six network elements NE1, . . . , NE6 receives messagesignals from one or from several of the other not illustrated networkelements, and transmits to them.

The six network elements NE1, . . . , NE6 are connected to a centralclock generator SASE and from it receive a reference clock REF to whicheach network element NE1, . . . , NE6 tunes its own clock generator. Inthis way the network elements NE1, . . . , NE6 work synchronously witheach other. In this embodiment, the central clock generator SASE in turnis connected to clock signal outputs of two of the network elements NE1,NE4 and receives clock signals 2M from them. Such a clock signal 2Mcontains a clock which was derived by the transmitting network elementNE1, NE4 from a received message signal STM-N, and also a qualityindicator which reflects the accuracy of the clock contained in themessage signal STM-N.

The clock signal in the first embodiment is a signal with a transmissionrate of 2 MBit/s, and the clock has a pulse frequency rate of 2 MHzwhich is derived from a message signal STM-N of the synchronous digitalhierarchy (SDH). The clock signal transmits the quality indicator (SSM)which is contained in the framework of the received message signal STM-Nin accordance with ITU-T recommendation G.707.

The central clock generator SASE selects one of the clock signals bymeans of the quality indicators, derives a clock from the selected clocksignal 2M and uses it to tune its internal clock provider. This internalclock provider represents the reference clock source of the network nodeNODE and the reference clock for the network elements NE1, . . . , NE6is obtained from it. In the event of a disturbance e.g. the breakdown ofparticular message signals from which the clock signals are derived, theclock provider of the central clock generator continues to function inthe open unsynchronized operation and continues to produce a referenceclock REF for the network node NODE. The accuracy of the reference clockin the open unsynchronized operation then corresponds to the accuracy ofthe clock provider, which in the embodiment is the accuracy establishedin the ITU-T G.812.

The control installation STE in the first embodiment is connected to thecentral clock generator SASE and from it receives a message STAT aboutits synchronization status. The content of this message is the accuracyof the selected clock according to the pertinent SSM and the origin ofthe clock, i.e. from which network element the selected clock signal isreceived and which of the clock signal inputs has the available clocksignal. The first embodiment uses an X.25 interface for the connectionbetween the central clock generator SASE and the control installationSTE.

The message STAT is always sent by the central clock generator SASE whenit selects another clock signal for adjusting its clock provider, forexample due to a breakdown of the selected clock signal. The time whenthis message is sent to the control installation is not critical,because a change in the accuracy of the reference clock REF only affectsthe synchronous digital communications system after a relatively longtime span of several hours or days. This is due to the specified highaccuracy of the received clocks and the high accuracy of the clockprovider in the open unsynchronized operation, which is established forexample for the SDH in the ITU-T recommendations G.811, G.812 and G.813.One effect on the synchronous digital communications system could be anincrease in the slip rate for example.

The control installation STE has a connection to the individual networkelements NE1, . . . , NE6 via which it sends instructions ANW to thenetwork elements NE1, . . . , NE6. This is indicated in FIG. 1 by aheavier line, which does not mean however that it is a parallelconnection via which all the network elements receive the sameinstruction, but that each network element receives one or several oftheir own instructions. This information can be transmitted via theexisting Q-interfaces of the network management, which are establishedin the ITU-T recommendations Q.811 and Q.812. The instructions ANWconcern the SSM which the network elements NE1, . . . , NE6 transmitfrom their individual outputs with the message signals STM-N they sendfrom these outputs.

FIG. 2 illustrates a second embodiment for a control installation STE.The illustrated network node NODE is the same network node NODE as inFIG. 1. The control installation STE in this second embodiment comprisestwo separate units: a first control unit SSU-M which is connected to thecentral clock generator SASE, and a second control unit SDH-M which isconnected to the network elements NE1, . . . , NE6. A transmissionmedium Q connects the two control units to each other. This isadvantageously a Q-interface. The first control unit SSU-M is a unit forcontrolling and monitoring a number of central clock generators ofdifferent network nodes. The second control unit is used for controllingand monitoring network elements of the synchronous digitalcommunications system.

FIG. 3. illustrates in detail one of the network elements NE1 and thecentral clock generator SASE from the first two embodiments. In thisexample the network element has four connector units IO1, . . . , IO4whereby message signals STM-N from other not illustrated networkelements are received and transmitted. Each of two connector units IO1,IO2 is assigned a clock derivation circuit TA which derives a clock fromthe received message signals STM-N. In addition the two connector unitsIO1, IO2 read the SSM of the received message signal STM-N. A clocksignal 2M which contains the SSM is generated from the derived clock.These clock signals 2M can be obtained from two clock signal outputs O1,O2.

The network element NE1 also has a reference clock input at which areference clock REF is received from the central clock generator SASE.The reference clock REF is routed to a clock provider SEC of the networkelement NE1 in order to tune it to the reference clock REF. The networkelement NEl can receive instructions ANW from a control installation viaa further input AI. The instructions concern the SSM to be transmittedby the individual connector units with the message signals STM-N

The central clock generator in the embodiment has six clock signalinputs I1, . . . , I6 which receive clock signals 2M from networkelements. In the illustrated example the two clock signal inputs I1, I2are connected to the two clock signal outputs O1, O2 of the networkelement NE1. The remaining clock signal inputs can be connected to theclock signal outputs of other network elements of the node. The clocksignal inputs I1, . . . , I6 are connected to a selector unit SEL. Itevaluates the SSM contained in the clock signals and uses the SSM toselect the clock signal with the most accurate clock. If several clocksignals have the same clock quality, either a specified ranking sequencecan be used for the selection, so that the clock signal of apredetermined clock signal input with the same quality is preferablyselected, or a random selection can be made.

A clock is derived from the selected clock signal and routed to a clockprovider GEN of the central clock generator SASE in order to tune thelatter. The clock provider GEN produces the reference clock REF for allthe network elements of the node and is connected to the reference clockoutput CO from where the reference clock can be obtained. This referenceclock output CO is connected to the reference clock input CI of thenetwork element NE1.

A synchronous digital communications system of a third embodimentillustrated in FIG. 3 contains three interconnected network nodes NODE1,NODE2, NODE3 and a control installation STE. Each of the three networknodes is made up of three network elements NE11, . . . , NE13, . . . ,NE31, . . . , NE33 and a central clock generator SASE1, . . . , SASE3.But the number of network elements in a network node is not limited tothree. The network elements of a network node receive a reference clockREF from the central clock generator of the network node via referenceclock lines. The central clock generator receives clock signals 2M fromthe network elements. Of the connections via which the central clockgenerators receive the clock signals, only those are shown which therespective central clock generator has selected for tuning its clockprovider.

The central clock generators SASE1, . . . , SASE3 are connected to thecontrol installation STE and send it messages STAT about theirsynchronization status. On the basis of these messages the controlinstallation sends instructions ANW to the individual network elementsNE11, . . . , NE13, . . . , NE31, . . . , NE33 regarding which SSM theyare to transmit to their individual outputs. In the embodiment, thecentral clock generator SASE3 of the third network node NODE3 hasselected a clock signal 2M from network element NE32 for synchronizingits clock provider. The clock contained therein comes from a notillustrated source to which the network element NE32 is connected viamessage connections STM-N, and has the accuracy established in the ITU-Trecommendation G.811. The central clock generator SASE3 informs thecontrol installation STE that it has selected the clock signal for thereference clock from network element NE32, and that this clock has theG.811 accuracy. The central control then gives instructions ANW to thenetwork elements regarding which SSM they are to transmit to theiroutputs. Table 1 lists which SSM the network elements send to theiroutputs.

TABLE 1 SSM transmitted by the individual network elements to thedifferent outputs. From: To: SSM NE11 NE21 DNU NE21 NE11 G.811 NE11 NE22DNU NE22 NE11 G.811 NE12 NE23 DNU NE23 NE12 G.811 NE13 NE31 DNU NE31NE13 G.811 NE31 NE23 G.811 NE23 NE31 DNU

The reference clock of the third network node NODE3 has the G.811accuracy. The network elements NE31, . . . , NE33 of the third networknode therefore transmit the code G.811 with the SSM. The network elementNE23 of the second network node receives a message signal with thestated clock quality from the network element NE31, derives a clocksignal from it and routes it to the central clock generator SASE2 of thesecond network node. The latter selects the clock signal as thereference clock for the second network node NODE2 and distributes theclock to the connected network elements NE21, . . . , NE23. Theinformation for the control installation includes the message that themessage signal used to synchronize the node comes from a predeterminedinput of the network element NE23, and that the accuracy of this clockis G.811. The control installation now informs the network element NE23that it must transmit the SSM DNU (Do Not Use for synchronization) tothe network element NE31, but transmit the SSM G.811 to the networkelement NE12. The network elements NE22 and NE21 are also instructed totransmit the SSM G.811.

The network element NE11 of the first network node NODE1 receives arespective message signal from network element NE21 and from networkelement NE22, that the SSM contains G.811. From each of these messagesignals it derives a clock signal for the central clock generator SASE1of the first network node NODE1. The central clock generator SASE1selects one of these clock signals and informs the control installationSTE of the selection and the accuracy of the selected clock signal. Thecontrol installation STE instructs the network elements NE11 and NE12 totransmit the SSM DNU in the direction of the second network node. Thenetwork element NE13 must also transmit the SSM DNU to the networkelement NE31 of the third network node NODE3 so that no clock loop cantake place, while the clock generator SASE3 of the third network nodeNODE3 selects a clock signal for tuning its clock provider, whichcontains the clock of the message signal received by the network elementNE13.

The control installation STE determines the instructions ANW on thebasis of the message STAT. To that end a memory in the controlinstallation can contain predetermined tables which include theinstructions for the individual network elements for every possiblecombination of messages from the connected central clock generator.Another possibility is for the central control to have information aboutthe configuration of the connected network nodes, i.e. which connectionsexist between the network nodes, and which inputs of the central clockgenerator contain a clock signal that is derived from message signalsreceived via these connections. By means of this information the centralcontrol can determine which SSM is to be transmitted by the individualnetwork elements.

What is claimed is:
 1. A control installation (STE) for a network node(NODE; NODE1, . . . , NODE3) of a synchronous digital communicationssystem containing several network elements (NE1, . . . , NE6; NE11, . .. , NE13, NE21, . . . , NE33) and a central clock generator (SASE;SASE1, . . . , SASE3), comprising: means for receiving a message (STAT)about a synchronization status of the central clock generator; and meansfor transmitting instructions (ANW) to the network elements, where theinstructions include quality indicators to be transmitted by the networkelements, and which are used to select an external reference clock fornetwork elements that do not belong to the network node.
 2. A controlinstallation (STE) as claimed in claim 1, which is provided forcontrolling several network nodes.
 3. A control installation (STE) asclaimed in claim 1, comprising a first control unit (SSU-M) and a secondcontrol unit (SDH-M) which are interconnected by a transmission medium(Q).
 4. A control installation (STE) as claimed in claim 1, where themeans for transmitting instructions (ANW) to the network elements (NE1,. . . , NE6) are means for sending information via a Q-interface.